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United States Patent |
6,052,160
|
Bohmer
,   et al.
|
April 18, 2000
|
Display device with sputter-resistant electrode layer for providing
plasma discharge
Abstract
The display device is furnished with a channel plate (39) comprising
channels (30, 30', 30") containing an ionizable gas (33), and the walls of
the channels (30, 30', 30") being provided with electrodes (31, 32) for
selectively generating a plasma discharge of the ionizable gas (33) during
operation. The display device further comprises an electro-optical layer
(35) of a material having an optical property which is governed by the
discharge state of the plasma discharge. The display device is
characterized in that at least one of the electrodes (31, 32) is furnished
with a layer (37) comprising particles of a sputter-resistant material
having an average diameter .ltoreq.2.5 .mu.m, preferably .ltoreq.1.5
.mu.m. The particles are preferably composed of rare-earth borides (for
example LaB.sub.6) or ruthenium oxide.
Inventors:
|
Bohmer; Marcel R. (Eindhoven, NL);
Scholten; Monica (Eindhoven, NL);
Van Slooten; Udo (Eindhoven, NL)
|
Assignee:
|
U.S. Philips Corporation (New York, NY)
|
Appl. No.:
|
111545 |
Filed:
|
July 8, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
349/32; 313/583 |
Intern'l Class: |
G02F 001/133 |
Field of Search: |
349/32
313/583,587,518,633
345/169.4
|
References Cited
U.S. Patent Documents
4599076 | Jul., 1986 | Yokono et al. | 445/24.
|
5783906 | Jul., 1998 | Moore et al. | 313/586.
|
5917284 | Jun., 1999 | Moore et al. | 313/586.
|
Foreign Patent Documents |
0762460A2 | Mar., 1997 | EP | .
|
0827176A2 | Mar., 1998 | EP.
| |
Primary Examiner: Dudek; James A.
Attorney, Agent or Firm: Fox; John C.
Claims
We claim:
1. A display device comprising
at least one compartment (30, 30', 30") containing an ionizable gas (33),
walls of said compartment (30, 30', 30") being provided with electrodes
(31, 32) for, in operation, selectively generating a plasma discharge of
the ionizable gas (33),
and an electro-optical layer (35) including a material having an optical
property which is governed by the discharge state of the plasma discharge,
characterized in that at least one of the electrodes is furnished with a
layer (37) comprising particles of a sputter-resistant material, said
particles having an average diameter below 2.5 .mu.m.
2. A display device as claimed in claim 1, characterized in that the
average diameter of the particles is below 1.5 .mu.m.
3. A display device as claimed in claim 1, characterized in that the
particles are composed of a rare-earth boride or ruthenium oxide.
4. A display device as claimed in claim 3, characterized in that the boride
is LaB.sub.6.
5. A display device as claimed in claim 1, characterized in that the layer
(37) further comprises a frit.
6. A display device as claimed in claim 5, characterized in that the frit
is a glass frit.
7. A display device as claimed in claim 1, characterized in that the
electro-optical layer (35) comprises a layer (35) of an electro-optical
material, and in that the display device is furnished with means which can
suitably be used to activate the electro-optical layer (35).
8. A display device as claimed in claim 7, characterized in that the
electro-optical material comprises a liquid-crystal material.
9. A display device as claimed in claim 1, characterized in that the
material of the electro-optical layer (35) comprises electroluminescent or
photoluminescent phosphors.
10. A display device as claimed in claim 2, characterized in that the
particles are composed of a rare-earth boride or ruthenium oxide.
11. A display device as claimed in claim 10, characterized in that the
boride is LaB.sub.6.
12. A display device of claim 10, characterized in that the layer (37)
further comprises a glass frit.
13. A display device as claimed in claim 2, characterized in that the
electro-optical layer (35) comprises a layer (35) of an electro-optical
material, and in that the display device is furnished with means which can
suitably be used to activate the electro-optical layer (35).
14. A display device as claimed in claim 13, characterized in that the
electro-optical material comprises a liquid-crystal material.
15. A display device as claimed in claim 2, characterized in that the
material of the electro-optical layer (35) comprises electroluminescent or
photoluminescent phosphors.
16. A display device as claimed in claim 1, characterized in that the layer
(37) is free of glass frit.
17. A display device as claimed in claim 2, characterized in that the layer
(37) is free of glass frit.
Description
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising at least one
compartment containing an ionizable gas, walls of said compartment being
provided with electrodes for, in operation, selectively generating a
plasma discharge of the ionizable gas, and an electro-optical layer
including a material having an optical property which is governed by the
discharge state of the plasma discharge.
Display devices for displaying monochromatic or color images include, inter
alia, plasma-addressed liquid-crystal display devices, the so-called PALC
displays and (direct-current) plasma-display panels (PDPs). The PALC
displays and PDPs are applied as television and computer displays and are
preferably of the thin type.
A display device of the type mentioned in the opening paragraph is
disclosed in European patent application EP-A 0 762 460. The thin-type
display device described in said document comprises a display screen with
a pattern of (identical) so-called data-storage or display elements and a
plurality of compartments. The compartments are filled with an ionizable
gas and furnished with electrodes for (selectively) ionizing the ionizable
gas during operation. In the known display device, the compartments take
the form of mutually parallel, elongated channels (formed in a so-called
channel plate), which serve as selection means for the display device (the
so-called plasma-addressed row electrodes). By applying a DC voltage
difference across the electrodes in one of the channels of the channel
plate, electrons are emitted (from the cathode), which ionize the
ionizable gas, thereby forming a plasma (plasma-discharge). If the voltage
across the electrodes in the one channel is switched off and the gas is
de-ionized, a subsequent channel is energized. At the displayscreen side
of the display device, the compartments are closed by a (thin) dielectric
layer ("microscheet"). The display device further comprises a layer of an
electro-optical material provided on a substrate and further electrodes
which serve as so-called data electrodes or column electrodes of the
display device. The display device is formed by the assembly of the
channel plate with the electrodes and the ionizable gas, the dielectric
layer, the layer of the electro-optical material and the further
electrodes.
In a plasma-display panel, a plasma discharge is used to directly excite a
layer containing electroluminescent phosphors of display elements, while
(electrons of) the ionized gas in the compartment itself excite(s) the
phosphors. In an alternative embodiment of a plasma-display panel, a
plasma discharge is used to generate light (for example UV light), said
light exciting a layer containing photoluminescent phosphors of display
elements.
A drawback of the known display device is that the energy consumption of
such display devices is still relatively high.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a display device
whose energy consumption has been further reduced.
To achieve this, the display device in accordance with the invention is
characterized in that at least one of the electrodes is furnished with a
layer comprising particles of a sputter-resistant material, said particles
having an average diameter below 2.5 .mu.m.
At the beginning of a plasma-discharge cycle in the display device, a
plasma discharge is created in the compartment or in one of the
compartments (for example in the channels of a PALC-display panel) by
applying a (relatively high) voltage pulse (the so-called "strobe"-pulse)
across the electrodes in the compartment. In such a plasma discharge,
charged particles are created. The voltage across and the current in the
discharge reach a stationary condition (the so-called "steady state")
within a few .mu.s. After switching off the plasma discharge, the grey
level of each display element is checked by applying a (relatively low)
voltage across the corresponding further electrode (the data electrode or
column electrode). As a result, a part of the charged particles is drawn
towards the (thin) dielectric layer ("microsheet"), causing the
electro-optical layer to be subjected to an electric field. As a result,
the electro-optical layer is charged like a capacitor until the full data
voltage is present across the layer, consequent upon which the
transparency of the electro-optical layer changes (for example, the layer
becomes more or less transparent). The degree of transparency, which is an
optical property of the electro-optical layer, is determined by the level
of the data voltage and hence depends on the discharge state of the plasma
discharge. After the discharge has disappeared, in the so-called
"afterglow", the compartment forms an insulator and the electro-optical
layer remains charged. A new plasma-discharge in the compartment effects a
reset or a renewed selection of the display element.
Important parameters of the plasma-discharge cycle of the display device
are the electric conductivity of the plasma discharge and the decay of the
conductivity in the afterglow period. If the decay in conductivity of the
plasma discharge takes too long, the discharge may continue while a
subsequent data line is already being written, which is undesirable. Too
rapid a decay of the conductivity also has adverse effects.
In general, the display device comprises a number of compartments, each
compartment incorporating at least two electrodes for ionizing the gas.
In the case of a plasma-addressed liquid-crystal display device (PALC
display), the electro-optical layer of the material having an optical
property which is governed by the discharge state of the plasma discharge,
comprises a layer of an electrooptical material (for example a
liquid-crystal material). Such a display device generally further includes
means which are suitable for activating the electro-optical layer. In a
PALC display, the ionized gas serves as a virtual switch for the
electro-optical material.
In the case of a (direct-current) plasma-display panel (dc PDP), the
electro-optical layer of the material having an optical property which is
governed by the discharge state of the plasma discharge, comprises
so-called electroluminescent or photoluminescent phosphors. In the
instance of electroluminescent phosphors, the ionized gas excites the
phosphors in the compartment itself, and in the case of photoluminescent
phosphors, the ionized gas emits light which causes the phosphors to emit
visible light (of the desired color).
As a (dc) plasma discharge is employed in the display device, the
electrodes in the compartment are subject to an ion bombardment, and the
electrodes, in general, are manufactured from a sputter-resistant material
or the electrodes are preferably provided with a sputter-resistant layer.
The use of a sputter-resistant material increases the resistance of the
electrodes against sputtering. In the known display device, this is
achieved by covering the electrodes (by means of electrophoresis) with a
coating of a hexaboride and a glass frit.
The inventors have recognized that the energy balance of the display device
can be improved by increasing the conductivity of the sputter-resistant
particles in the layer. A problem of the known layer deposited on (one of)
the electrodes in the compartment of the display device resides in that
the conductivity of the sputter-resistant particles in the layer is
generally insufficient. Ignition of a plasma in the known display device
in which (one of) the electrodes is/are provided with the known layer
causes brightly illuminating anode spots.
By providing, in accordance with the invention, at least one of the
electrodes with a(n) (electroconductive) layer comprising (microscopic)
particles of a sputter-resistant material (refractory material) having an
average particle diameter below 2.5 .mu.m, it is achieved, under otherwise
equal conditions, that the effective surface of the particles increases
(leading to a reduction of the effective work function) as well as the
contact surface between the particles, so that charging of the particles
is effectively reduced. By virtue of the measure in accordance with the
invention, the work function of the layer comprising sputter-resistant
particles is reduced and a good electroconductive layer is obtained. An
average particle diameter of 2.5 .mu.m is to be taken to mean in this
application that d.sub.50 =2.5 .mu.m, the subscript "50" indicating that
50% by weight of the diameters of the particles is smaller than or equal
to said value. In the case of a typical grain size distribution and a
value of d.sub.50 =2.5 .mu.m, the corresponding values for d.sub.10 and
d.sub.90 are respectively .about.1.7 .mu.m and .about.3.7 .mu.m, the
definitions of d.sub.10 and d.sub.90 corresponding to that of d.sub.50.
In the known display device, the particles used are "typically about 4.0
microns in diameter". Particles of such a size provided by means of
electrophoresis exhibit a conductivity which is so low that the addition
of a considerable quantity of glass frit is necessary to obtain the
desired conductivity. In accordance with the inventive measure, a
satisfactorily conducting, sputter-resistant layer can be used which
includes more sputter-resistant material. An additional advantage of the
omission or reduction of the quantity of glass frit is that such a layer
comprising sputter-resistant particles has a high secondary
electron-emission coefficient, causing the ignition and sustain voltages
and hence the energy consumption of the display device to be reduced.
The above-mentioned anode spots do not occur in a layer provided on the
electrodes in the compartment of a display device having a particle
diameter in accordance with the invention. In addition, the improved
conductivity of the sputter-resistant material enables the electro-optical
layer to be better addressed. An additional advantage is that a layer
having such a relatively small particle size can also be much thinner.
When the electrode is (electrophoretically) covered with such an optically
non-transparent layer, said layer also grows partly around the electrode,
so that a thinner layer also leads to a lower aperture loss in the display
device.
Preferably, the average diameter of the particles is below 1.5 .mu.m. The
work function can be further reduced and the conductivity further improved
by providing (one of) the electrode(s) with a layer of particles having an
average particle diameter (d.sub.50 .ltoreq.1.5 .mu.m).
Particles which can suitably be used to achieve the object of the invention
are rare-earth borides, for example LaB.sub.6 or GdB.sub.6, or ruthenium
oxide (RuO.sub.2).
In an alternative embodiment in accordance with the invention, a (glass)
frit is added to the layer of relatively small particles of a
sputter-resistant material applied to (at least one of) the electrodes in
the channels of the display device. The adhesion of the layer is improved
by adding, during the manufacture of the (coating) layer, a preferably
very finely ground glass frit (typical grain size .ltoreq.1 .mu.m) to the
particles of the sputter-resistant material.
In the known display device, the glass frit is provided between and
(partly) on the particles (see FIG. 9 of EP-A 0 762 460) in order to bond
the particles together, which leads to a reduction of the active surface
of the particles. In the known layer, the frit is employed predominantly
to enlarge the contact surface between the particles of the
sputter-resistant material. Said frit causes the active surface of the
sputter-resistant particles to be reduced. The addition of a (glass) frit
in accordance with the inventive measure serves to improve the adhesion of
the sputter-resistant particles to the surface of the electrodes and
generally does not lead to a reduction of the active surface of the
sputter-resistant particles, which would be undesirable in view of the
fact that the secondary emission coefficient of the sputter-resistant
particles should be as high as possible.
These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 shows a schematic block diagram of a display device;
FIG. 2 is a schematic, perspective view, partly cut-away, of a part of a
construction of a plasma-addressed liquid-crystal display device (PALC);
FIG. 3 shows the resistance of LaB.sub.6 powder as a function of the
relative packing density for different particle sizes.
The Figures are purely schematic and not drawn to scale. In particular for
clarity, some dimensions are exaggerated strongly. In the Figures, like
reference numerals refer to like parts, whenever possible.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 very schematically shows a block diagram of a conventional display
evice. Said display device comprises a substrate 1 having a surface 2
provided with a pattern of pixels separated from each other in the
vertical and horizontal directions (the space between the pixels being
predetermined). Each pixel 3 comprises overlapping portions of (thin,
narrow) electrodes 4 of a group of electrodes arranged in vertical columns
and (thin, narrow) electrodes 5 of a further group of electrodes arranged
in horizontal rows. The electrodes 4 of the group of electrodes are also
referred to as column electrodes, and the electrodes 5 of the further
group of electrodes are also referred to as row electrodes. In a
plasma-addressed liquid-crystal display device (PALC), the rows are formed
by long, narrow channels (the compartments). The pixels 3 in each of the
rows of electrodes (channels) 5 represent one data line.
The width of the electrodes 4, 5 determines the dimensions of the pixels 3,
which are typically rectangular in shape. Electrodes 4 receive (analog)
drive signals ("data drive signals") from a drive circuit 8 via parallel
conductors 6, and electrodes 5 receive (analog) drive signals ("data drive
signals") from a drive circuit 8' via parallel conductors 7.
To produce an image or a data-graphic display on a relevant area of the
surface 2 of substrate 1, the display device employs a control circuit 8"
("scan control circuit"), which controls the drive circuits 8, 8'. In the
display device, various types of electro-optical materials may be used.
Examples of electro-optical materials include (twisted) nematic or
ferro-electric liquid-crystal materials. In general, the electro-optical
materials weaken the passed or reflected light in dependence upon a
voltage applied across the material.
FIG. 2 is a schematic, perspective view, partly cut away, of a part of a
construction of a plasma-addressed liquid-crystal display device (PALC)
comprising a first substrate 38 and a second substrate 39. In FIG. 2, only
three column electrodes 29, 29', 29" are shown. The row electrodes 30,
30', 30", which serve as selection means, are formed by a number of
mutually parallel, elongated channels (compartments) below an
electro-optical layer 35 of an electro-optical material. The panel is
provided with electric connections to the column electrodes 29, 29', 29"
and to the plasma electrodes 31, 32, said column electrodes 29, 29', 29"
receiving (analog) drive signals from output amplifiers 27, 27', 27", and
the anode electrodes 32 in the (plasma) channels 30, 30', 30" receiving
drive signals from output amplifiers 26, 26'. Each of the (plasma)
channels 30, 30', 30" is filled with an ionizable gas 33 and is sealed
with a thin dielectric layer ("microsheet") 36 which is made, for example,
of glass. Each of the compartments (the channels) is provided at an inner
surface (wall) with first and second elongated electrodes 31, 32 extending
throughout the length of the channel. The second electrode 32 is referred
to as the anode and is supplied with a pulsed voltage, a so-called "strobe
pulse", causing electrons emitted from the cathode 31 to ionize the gas,
thereby forming a plasma. In an alternative embodiment, a negative
(direct-current) pulse is applied to the cathode. The next channel is not
energized until after the "strobe pulse" has ended and the gas has been
de-ionized. To reduce the duration of the cycle, the next channel is
generally ionized already before the preceding channel has been
(completely) de-ionized. The column electrodes 29, 29', 29" each cross an
entire column of pixels, so that, in order to preclude crosstalk, the
number of plasma row connections per unit of time is limited to only one.
In accordance with the invention, at least one of the electrodes 31, 32 is
furnished with an (electroconductive) layer 37 of (microscopic) particles
of a sputter-resistant material (refractory material) having an average
particle diameter below 2.5 .mu.m (d.sub.50 .ltoreq.2.5 .mu.m). The layer
37 is preferably provided by means of electrophoresis. By employing
particles having an average diameter below 2.5 .mu.m, the desired
conductivity of the conductive layer 37 is achieved without the necessity
of adding a glass frit to the layer. By virtue of the absence of the glass
frit, such a layer has a higher secondary electron-emission coefficient,
which causes the ignition and sustain voltages and hence the energy
consumption of the display device to be reduced considerably. In a layer
37 which is applied to the electrodes 31, 32 in the (plasma) channels 30,
30', 30" of the display device and which includes particles having a
diameter in accordance with the invention, anode spots do not occur. In
addition, the improved conductivity of the sputter-resistant material
enables the electro-optical layer to be better addressed. By virtue of the
relatively small particles, the layer 37 is generally thinner, resulting
in a lower aperture loss in the display device. Preferably, the average
diameter of the particles in the layer 37 is below 1.5 .mu.m (d.sub.50
.ltoreq.1.5 .mu.m), with d.sub.90 .ltoreq.2.4 .mu.m. Suitable materials
include rare-earth borides, for example LaB.sub.6 or GdB.sub.6, or
ruthenium oxide (RuO.sub.2). Other suitable materials include Cr.sub.3 Si,
diamond, diamond-like carbon and barium tantalate (Ba.sub.4 Ta.sub.2
O.sub.9).
FIG. 3 shows the resistance (R in .OMEGA.m) of various LaB.sub.6 powders
(in this case, two types of commercially available powders) as a function
of the relative packing density (.rho.in %) for different particle sizes.
The open triangles and open squares show the resistance R of the known
LaB.sub.6 particles having an average size of 4 .mu.m (ar="as received").
This powder only becomes conducting at a packing density in excess of 50%.
Such a packing density is not achieved in a standard electrophoretic
coating process, so that a layer comprising sputter-resistant particles
having an average diameter of 4 .mu.m is not conducting. A layer
comprising particles having an average diameter of 4 .mu.m generally only
reaches the desired conductivity by adding an (electroconductive) glass
frit to the layer.
In FIG. 3, the closed triangles and closed squares represent the resistance
R of the known LaB.sub.6 particles having an average size of 1.5 .mu.m
(m="milled"). In this example, such a particle size is obtained by
grinding the "as received" LaB.sub.6 particles (4 .mu.m) on a roller
machine with 2 mm Si.sub.3 N.sub.4 balls for 24 hours. The resultant
powder (d.sub.50 .about.1.5 .mu.m) is conducting already at a packing
density above 40%. Such a packing density is readily achieved in a
standard electrophoretic coating process, and causes a layer comprising
sputter-resistant particles having an average diameter of 1.5 .mu.m to be
electrically conducting. A layer comprising particles having an average
diameter of 1.5 .mu.m, which is applied by means of a standard
electrophoretic coating process, has the desired conductivity without a
glass frit having been added to the layer. To improve the adhesion of the
sputter-resistant particles to the contact surface of the electrode, a
relatively thin layer of a glass frit may be added without the active
surface of the sputter-resistant particles necessary for an optimum
secondary electron emission being effectively reduced.
It will be obvious that, within the scope of the invention, many variations
are possible to those skilled in the art.
In general, the invention relates to a display device comprising a channel
plate (39) including channels (30, 30', 30") containing an ionizable gas
(33), and walls of the channels being provided with electrodes (31, 32)
for selectively ionizing the ionizable gas (33), during operation. The
display device further comprises an electro-optical layer (35) of a
material having an optical property which is governed by the discharge
state of the plasma-discharge. The display device is characterized in that
at least one of the electrodes is provided with a layer (37) comprising
particles of a sputter-resistant material having an average diameter
.ltoreq.2.5 .mu.m, preferably .ltoreq.1.5 .mu.m. The particles are
preferably composed of rare-earth borides (for example LaB.sub.6) or
ruthenium oxide.
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